The present invention relates to shock sensing in a dual-actuator servo system for hard disk drives. More particularly, the present invention is directed to use of a piezoelectric element for both positioning of the actuator and for sensing shocks imparted to the hard disk drive assembly.
A hard disk drive typically includes one or more magnetic disks rotatably mounted in association with a spindle and one or more actuators for positioning a transducer or head relative to data tracks on the surface of each disk. The actuator typically comprises a pivotable base plate, a load beam or suspension arm affixed to the base plate, a flexure at the distal end of the load beam and a slider and transducer or a head assembly affixed to the flexure. A voice coil motor induces movement of the actuator to position the head relative to the disk surface. Typically, the voice coil motor and servo system provide both gross positioning of the head, i.e., track to track positioning, and fine positioning, i.e., track following. Recently, piezoelectric elements have been incorporated into actuators to provide fine positioning of the head assembly for track following purposes, rather than only relying upon the voice coil motor for fine positioning.
With the emphasis on making hard drives smaller, for numerous applications including portable computers, magnetic disks are not only becoming smaller, but data tracks are becoming increasingly more densely positioned on the disks and the tracks themselves are becoming physically narrower. As a result, maintaining the transducer or head in an accurate track following position for purposes of reading and writing is becoming more complex. To accommodate increasingly finer adjustments in the position of the magnetic head, dual positionable actuators have been introduced. In a first mode, a voice coil motor will move the actuator from track to track. In a second mode, the voice coil motor will provide fine positioning of the distal end and head assembly, together with a piezoelectric element positioned on the actuator. By supplying a current to the piezoelectric element, the piezoelectric element can expand and contract in a controlled manner to adjust the distal end of the actuator arm and thereby accurately maintain position of the head relative to the disk surface.
Piezoelectric elements are also used as shock sensors in disk drives. Specifically, as applications for disk drives place the drives in mobile environments, such as with portable computers, the chance of external shock being imparted to the disk drive is increasing. In this context, an external shock may be strong enough to dislodge the head from its intended position. If this were to occur during a read operation, an error would result and the read operation would need to be redone. However, if a shock moves a head during a write operation, a more significant problem can occur. It is conceivable that the information being written could be written to a wrong location. Therefore, it is imperative to be able to shut down a write operation before the event could occur. The present invention addresses and solves this problem by utilizing a piezoelectric shock sensor to determine the presence of a shock and, if necessary given the magnitude of the shock, to abort write operations.
The present invention combines the functionality of actuator positioning and shock sensing into a common element. Direct measurement of shock during track following improves performance and reduces the possibility of track encroachment, i.e. writing information to an incorrect or unintended location. By utilizing common piezoelectric elements for both track following and shock sensing, the present invention lowers the cost of disk drives by removing superfluous elements, together with unnecessary and detailed wiring as would be the case with a separate shock sensor and actuator positioning element.
The present invention advantageously utilizes the physics of piezoelectric elements in which movement is generated when a charge is applied across a piezoelectric element and conversely, a voltage is generated when a motion is forced on the piezoelectric element. These characteristics allow piezoelectric elements to be used as both actuators and sensors simultaneously. Although the preferred embodiment disclosed is described in terms of controlled charge and sensed voltage, a person of skill in the art will recognize that voltage could be controlled and charged sensed with the same result. The present invention is not intended to be limited to a charge controlled piezoelectric system. Rather, it is intended to cover any use of a common piezoelectric element or its equivalent.
A piezoelectric element is a crystal or quartz structure. Compression of the element generates an electrostatic voltage across it and conversely, application of an electric field causes the crystal to expand or contract. The polarization of a piezoelectric crystal is parallel to the axes of the crystal. The magnitude of the polarization is proportional to the stress and/or strain (compression and/or tension) applied to the crystal. Importantly, the direction of the polarization is reversed when the strain changes from compression to tension, and vice versa. For example, if a piezoelectric element is subjected to an alternating current, it will alternately expand and contract. Conversely, subjecting the piezoelectric element to a tension and compression force will generate a voltage, but in opposite directions.